1. Field of the Invention
The present invention relates to the field of blood pressure measuring instruments or sphygmomanometers; more particularly, it relates to such instruments utilizing acoustical pickups or microphones for determining, by blood sounds, the pressure levels at which systolic and diastolic blood pressure are to be measured.
2. Description of the Prior Art
Periodic and accurate blood pressure determinations are essential to proper diagnosis of heart function, and particularly, to discover hypertension in humans. Instruments used to measure blood pressure are called sphygmomanometers, the most common type of such instruments being an inflatable cuff which is secured around a patient's upper arm. The operator listens, normally via a stethoscope positioned against the patient's artery in the lower arm, to the heartbeat and inflates the cuff until the heartbeat no longer can be heard. Blood flow is then effectively cut off to the lower arm. The pressure in the cuff is at all times registered on a pressure gauge, generally a mercury manometer. When the blood flow to the lower arm is completely occluded, pressure is slowly released from the cuff. The cuff pressure at which the operator again just begins to hear characteristic blood flow noises (Korotkoff sounds) is termed the systolic blood pressure of the patient. Additional pressure is released from the cuff until the Korotkoff sounds momentarily get louder and then fade. The point at which the Korotkoff sounds first disappear is termed the diastolic blood pressure.
The described method requires relatively subjective determination of sound levels by the operator. Various extraneous factors, such as ambient noise and the operator's hearing, may affect the determination of exactly when the Korotkoff sounds can first be heard as the cuff is deflated and exactly when they later disappear. As accurate and repeatable blood pressure determinations may be essential to a proper determination of the patient's well being, improved and less subjective methods for determining blood pressure are desirable.
In order to eliminate subjective sound determinations by a stethoscope, cuff-type sphygmomanometers have sometimes been modified by insertion of a microphone in the cuff, the microphone being positioned over a restricted brachial artery when the cuff is fastened about a patient's arm. The microphone, which replaces the stethoscope, is electronically connected to an indicator, for example a light, and the cuff is inflated as above described until the blood supply is cut off to the patient's lower arm; then pressure is slowly released from the cuff. At the systolic pressure, when blood flow just resumes, the microphone detects the Korotkoff sounds and causes the light (or other indicator) to be actuated. As the cuff is further deflated, the Korotkoff sound is picked up at each pulse beat and, in typical systems, a light flashes at each beat. When diastolic pressure is reached, there is no longer a restriction of the blood vessels, and consequently no further Korotkoff sounds are picked up. At this point, flashing of the light stops. Systolic blood pressure is read from the manometer when the light first starts flashing, and the diastolic pressure is read when the light stops flashing.
A principal difficulty with the use of a microphone to pick up blood sounds is that background or extraneous noises arising, for example, from movement of the patient's arm or from the surrounding environment are also picked up by the microphone. Erroneous indications, and hence erroneous blood pressure readings, thus often occur.
Gilford, U.S. Pat. No. 2,827,040, discloses apparatus wherein blood sounds picked up by a microphone are correlated with blood pressure pulses caused by the heartbeat. An indicator is to be actuated only when picked-up sounds occur simultaneously with pressure pulses. This is intended to screen out extraneous noise sources. If, however, a background noise is picked up coincidentally with a pulse beat an erroneous indication will be given. Further improvement over such a system is needed.